My invention may be described in connection with aeration in wastewater treatment plant, but it is to be understood that the invention applies to gas-liquid contacting processes in general.
Surface aerators are usually rotated by a central shaft. Structurally the two widely applied surface aeration apparatus are (1) propeller type which have blades with various pitch extending radially from a center shaft and (2) so-called turbine type aerators which have curved blades on conical walls such as the one described in U.S. Pat. No. 3,576,316. These surface aeration devices attain gas-liquid contacting by continuously agitating and pumping liquid near liquid surface in a wastewater aeration tank. Hydrodynamically these surface aeration devices typically create surface jet layer flowing radially outward from these aerators. This inherently turbulent surface jet provides quick surface renewal mechanism for gas-liquid contacting, thus achieving mass transfer of the gas into the liquid body. However, due to the speed of the jet, a lump of fluid remains near the surface very short time and quickly flow into the liquid under the surface carrying a great portion of the kinetic energy, which is used for mixing and suspending solids in wastewater. This portion of the kinetic energy is rather well distributed throughout the tank and subsequently dissipated in the form of internal energy without contributing much to the gas-liquid mass transfer.
One of the most important requirements in aerating wastewater is the continuous suspension of solids in an aeration basin. To achieve this suspension, surface aerators have to provide sufficient energy of agitation. Considering the hydrodynamic flow pattern generated by typical surface aerators, the bottom scouring action, thus the ability to keep solids suspended, is limited by the nature of the recirculating turbulent flow in the particular basin. Consequently the input energy per liquid volume and geometry of the basin play a dominant role in solids suspension as well as gas-liquid mass transfer. In other words the overall effectiveness of the surface aeration methods developed so far is dominated by the nature of flow pattern which is dependent on the particularity of a basin geometry. Therefore instead of hoping to achieve drastic improvement in overall performance by merely manipulating certain portion of existing surface aerators construction, a new improved method and device have to be developed based on novel ideas with cost engineering in mind.
When the aeration basin is relatively deep, surface aerators can generate only weak bottom scouring action which cannot prevent the sedimentation of solids on the bottom. To prevent this problem, bottom mixing devices have been added. Typically an additional impeller is attached to the center shaft extended deep into the aeration basin. This arrangement not only complicates the mechanical structure but also requires additional power consumption. Moreover bottom mixing devices may change the circulating flow pattern such a way that surface renewal for gas-liquid contacting becomes much less efficient resulting in reduction in aeration efficiency.
Methods have been considered, such as described in U.S. Pat. No. 3,827,679, to solve the bottom mixing problem with surface aeration system as well as to obtain high efficiency in aeration by inserting a gas under pressure underneath the bottom mixer. However the introduction of a gas under pressure requires gas compressors which consume additional energy in addition to an increase in initial capital cost. These additional equipments, such as compressor and drive units, require more maintenance effort. Therefore attaching a bottom mixing impeller merely to solve sedimentation problem is not very desirable.
From a dynamic point of view, the turbulent energy requirement of a fluid for proper mixing is related to physical properties of the fluid, turbulence length scale created by a particular agitating device and turbulent intensity which has dominant effect on rate of decay of kinetic energy. The turbulent intensity can be interpreted as fluctuating flow velocity and will affect the mass transfer of gas into liquid on gas-liquid interface. An energy efficient aeration method has to incorporate all these factors to produce the best mass transfer mechanism.
It is essential to balance these physical phenomena to produce most energy economical aeration as well as to produce favorable flow configuration for good mixing and solid suspension. Moreover, for practical applications, maximum mechanical simplicity and minimum maintenance in operation is a very important factor. The present invention is based on the above considerations, and it's salient features are briefly explained below.
As described above, mixing energy is present in an aeration basin in the form of turbulent eddies which constitutes a very significant part of total energy input into the basin. Since, in surface aeration only, gas entrainment is mostly confined near the surface, a large amount of turbulent energy inside the basin is not efficiently utilized in aeration. The present invention, on the other hand, makes use of this energy in aerating the liquid by providing gas bubbles distributed throughout the basin, thus achieving high gas-liquid mass transfer efficiency.
To entrain gas bubbles, the following system is designed based on the principle of maximum mechanical simplicity and practicality. An impeller, mounted on a single shaft with a surface aerator, pumps liquid downward with such a speed that the pressure in the down flow liquid region becomes low enough to suck gas from the gas space through at least a duct. Then the entrained gas and liquid form two-phase flow which is pushed down through a guide tube and is discharged near the bottom of the basin. Subsequently this two-phase flow forms wall jet along the bottom floor and provides a very strong bottom scouring action. This favorable flow configuration along the bottom makes solids suspended, and also allows us to make aeration basin relatively deep which is often desirable in many practical situations. Moreover, the mechanical simplicity in the construction of surface aerator and down pumping impeller combination requires almost no additional maintenance in operation to the surface aeration only. In essence the most novel features of the present invention come from the concept of interaction mechanism of surface aeration and self-entrained gas bubbles, and the simplistic device which carries out this mechanism into effect.